1. Field of the Invention
The present invention relates to a condenser microphone unit and a condenser microphone including the condenser microphone unit, the condenser microphone unit preventing a variation in tension of a diaphragm even if a diaphragm ring is fixed through a fixing ring and securely making an electric connection between the diaphragm ring and the fixing ring.
2. Related Background Art
Main components of a condenser microphone unit include a diaphragm supported by a diaphragm ring and a fixed electrode provided opposite to the diaphragm with a gap corresponding to the thickness of a spacer provided therebetween, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-072271.
A conventional condenser microphone unit is illustrated in FIGS. 4 to 6. The conventional example depicts a unidirectional condenser microphone unit having two diaphragms in the front and back.
With reference to FIG. 4, a condenser microphone unit 1 has a unit casing 15 including an insulating washer 6 that divides the inside thereof into two sections. A front microphone element 20 is provided in the front (upper side in FIG. 4) of the insulating washer 6 and a back microphone element 30 is provided in the back (lower side in FIG. 4) thereof.
The microphone element 20 has a front sound terminal (not shown in the drawing) provided in the unit casing 15, a front diaphragm ring 4 accommodated in the unit casing 15, a front diaphragm 11 held by the front diaphragm ring 4, a front fixing ring 41 fixing the front diaphragm ring 4 to the unit casing 15, a front fixed electrode 8 provided opposite to the front diaphragm 11 through a spacer 46 with a predetermined gap therebetween, and a front air chamber 18 provided in the back of the front fixed electrode 8.
The microphone element 30 has a back sound terminal (not shown in the drawing) provided in the unit casing 15, a back diaphragm ring 5 accommodated in the unit casing 15, a back diaphragm 12 held by the back diaphragm ring 5, a back fixing ring 51 fixing the back diaphragm ring 5 to the unit casing 15, a back fixed electrode 9 provided opposite to the back diaphragm 12 through a spacer 47 with a predetermined gap therebetween, and a back air chamber 19 provided in the front of the back fixed electrode 9.
The front fixing ring 41 is a thin ring plate having a plurality of screw holes 43 to allow the front diaphragm ring 4 to be fixed by screws 42 without requiring screw holes directly in the front diaphragm ring 4. The front fixing ring 41 is fixed to the unit casing 15 with the screws 42. Then, the internal end portion of the front fixing ring 41 urges and fixes the front diaphragm ring 4 to the front fixed electrode 8. The front diaphragm ring 4 is thus indirectly fixed to the unit casing 15. Similar to the front diaphragm ring 4, the back diaphragm ring 5 is indirectly fixed to the unit casing 15 by the back fixing ring 51. The front fixing ring 41 and the back fixing ring 51 are each composed of a metal plate to provide conductivity.
The insulating washer 6 has a sound hole 7 to connect the front air chamber 18 and the back air chamber 19. Although only one sound hole 7 is depicted in FIG. 4, a plurality of sound holes 7 is provided in practice.
In such a conventional condenser microphone unit, sound waves received through the back sound terminal pass through the sound holes 7 in the insulating washer 6. The sound waves are then directed to the front air chamber 18 in the back of the front fixed electrode 8 and reach the back portion of the front diaphragm 11 to provide unidirectivity to the diaphragm 11. The front air chamber 18 in the back of the front fixed electrode 8 provides an omnidirectional driving force, while the sound waves from the back sound terminal provide a bidirectional driving force. At the bidirectional driving force equal to the omnidirectional driving force, unidirectivity is achieved where the sound collection direction is cardioid. The principal to achieve unidirectivity in the back diaphragm 12 is the same as that in the above-described front diaphragm 11 except that the relevant components are reversed between the front and back, thus the explanation is omitted.
In the condenser microphone unit described above, the opposing diaphragm and fixed electrode define a capacitor. The diaphragm vibrates in response to sound waves. Then, the gap between the diaphragm and the fixed electrode varies, thus varying the capacitance of the capacitor. The variation in capacitance is output as audio signals associated with variable voltage. The diaphragm is fixed to the diaphragm ring and accommodated under an appropriate tension in the unit casing.
A variation in tension of the diaphragm causes a variation in frequency response of the microphone unit. A main factor to cause the variation in tension of the diaphragm is a mechanical stress exerted on the diaphragm ring in the assembled microphone unit.
In particular, in the case where the diaphragm ring is indirectly fixed to the unit casing using the fixing ring fixed by the screws as described above, the urging force from the fixing ring is exerted unevenly on the diaphragm ring. Such uneven urging force may vary the tension of the diaphragm. Depending on the urged position of the diaphragm ring by the fixing ring, the diaphragm ring may be deformed or tilted inward or outward, thus causing a variation in tension of the diaphragm.
Furthermore, the diaphragm ring is fixed to the diaphragm and functions as an electrode provided opposite to the fixed electrode. Thus, the diaphragm ring and the fixing ring should be securely connected electrically. As described above, however, the uneven urging force from the fixing ring to the diaphragm ring may result in an unstable electric connection.